Up to the development of low pressure methanol synthesis processes dimethylether has been obtained in a quantity of 2-5 weight-% as a side-Product in high-pressure methanol production units and has been isolated by destillation from mixtures, which contained additional low boiling side-products.
After introduction of low pressure methanol processes, which yield only negligible quantities of dimethylether, special synthetic processes have been developed, based on the catalytic dehydration of methanol. Numerous processes have been disclosed in the patent literature. For example, according to DE-PS 680 328, aliphatic ethers are obtained by heating alcohols in the presence of zinc chloride.
Other suitable catalysts for the production of ethers from alcohols are according to GB-PS No. 332 756, GB-PS No. 350 010, GB-PS No. 403 402, U.S. Pat. No. 1,873,537 and FR-PS No. 701 335, ferrous and ferric chloride, copper sulfate, stannic and stannous chloride, manganese chloride, aluminum chloride and -sulfate, chromic sulfate, alum, thorium compounds, aluminum oxide, titanium oxide, barium oxide, silica or aluminum phospate.
In "Industrial and Engineering Chemistry", Vol. 41, No. 12, page 2928 (1949) use of bauxite with a SiO.sub.2 portion of 4.40 13.99 weight-% is described. In U.S. Pat. No. 3,036,134 an aluminum silicate-catalyst is disclosed for the production of dimethylether from methanol, with a ratio of Al.sub.2 O.sub.3 :SiO.sub.2 of 1 part: 1,35-0,3 parts. The synthesis of dimethylether directly from synthesis gas (CO+H.sub.2) has also been described (DE-PS No. 23 62 944, DE-PS No. 27 57 788 and DE-PS No. 32 20 547).
The technically most important catalysts have turned out to be according to DE-PS No. 28 18 831, DE-OS No. 32 01 155, EP-A No. 0 099 676 and EP-A No. 0 124 078 in particular, aluminum oxide and aluminum silicate catalysts with and without doping. In DE-PS No. 28 18 831 a catalyst for the production of dimethylether is disclosed, which can contain any aluminum oxide as a base material, as far as it possesses a sufficiently large surface and additives of 1 to 30 weight-% of rare earthes.
Finally in EP-A No. 0 099 676 a catalyst is disclosed, which contains 1-20 weight-% of SiO.sub.2, preferably 1-10 weight-% of SiO.sub.2 and more preferably 6 weight-% of SiO.sub.2.
Crude dimethylether thus obtained contains reaction water, unreacted methanol as well as small quantities of contaminations, like methyl formate, hydrocarbons, amines and sulfides, carboxylic acids and esters, amides, acetales and others.
In synthesis units of the state of the art, crude dimethylether is worked up in two distillation columns connected in series. In the first column, which operates under pressure, pure dimethylether is obtained. In the second one unreacted methanol is distilled off. Thus in EP-A No. 0 124 078 a process is described, according to which dimethylether is drawn off in a first column, which is operated under pressure as a sidestream, where as in a second column, which is operated under lower pressure, contaminations with boiling points between dimethylether and methanol, are drawn off overhead methanol is obtained in the same column as a sidestream. Catalysts are Al.sub.2 O.sub.3, SiO.sub.2, aluminum silicates and preferably .gamma.-Al.sub.2 O.sub.3. The use of .gamma.-Al.sub.2 O.sub.3 as a dehydration catalyst is known for many years. (Catalysis, Vol. VII, Ed. P.H. Emmett, Reinhold Publishing Corp., New York (1960), page 140).
Since dimethylether gains increasing importance as a propellant for sprays, very high demands are made with regard to purity. Thus, no irritating substances in dimethylether are permitted in applications like cosmetic, human and household sprays. Furthermore dimethylether has to be free of odour for these applications.
Object of the instant invention therefore was the production of a highly pure dimethylether by and to convert methanol nearly quantitatively into a highly pure product which is suitable for the above named applications.
Applicant has investigated numerous catalysts with regard to their suitability for the production of pure odorless dimethylether, which can be obtained in an economical distillative purification process Applicant has found that most catalysts, including Al.sub.2 O.sub.3, .gamma.-Al.sub.2 O.sub.3, aluminum silicates, silica and others lead to small quantities of strongly smelling substances, which either have to be separated by extensive purification or which can not be separated even by these processes. Although in many cases the constitution of these substances is not known, because they can not be identified by analytical methods, it is known that the starting material, which is refined methanol, contains already numerous contaminations, like dimethylamin, methyl mercaptane, formic acid, formid acid methyl ester, formaldehyde, formaldehyde dimethylacetate, acetic acid and others. These compounds respectively combinations of these compounds may even in traces lead to unpleasant odour in dimethylether.
Applicant has succeeded in solving the problems described above by the instant inventive process. According to the invention highly pure dimethylether is continuously produced by catalytic dehydration of methanol a temperature of 140.degree.-500.degree. C. and a pressure of 1-50 bar in the presence of .gamma.-Al.sub.2 O.sub.3 -catalyst, which contains a small quantity of SiO.sub.2 and distillative workup of the dehydration product.
Applicant has non-obviously found that the inventive catalyst leads to crude dimethylether which can be economically purified by distillation, whereby an odorless dimethylether is obtained in nearly quantitative yield. In the pressure of the inventive catalysts the formation of smelling contaminations is suppressed.
According to the state of the art, the determination of substances with an unpleasant odour is carried out predominantly by empirical methods, in particular by sensory determination by a trained team. Thus, the limit of inconvenience in case of H.sub.2 S has for example been identified in the Federal Republic of Germany by 150 persons as 45 .mu.g/m.sup.3. (Schriftenreihe der Landesanstalt fur Immissionsschutz des Landes Nordrhein-Westfalen, Heft 49 (1979), page 77). In those cases, when the limit of perception of odour can be analyzed by instruments, gaschromotography, electric conductivity photometry or fluorescence measurement are applied ("Erdol und Kohle-Erdgas-Petrochemie, Vol. 32, Nr. 2, Feb. 1975, page 86). The determinations of odour in the instant application are based on sensory methods.
Crude methanol from methanol synthesis units as well as dimethylether produced catalytically from methanol, contain as outlined above, numerous contaminations, with, in some cases, strong odour.
Crude dimethylether which is produced from crude or pure methanol in dimethylether synthesis units, consists of 20 to 80 weight-% of dimethylether and in addition the above named contaminations, reaction water and unconverted methanol. Since the boiling points of the contaminations, for example of dimethyl amine (b.p. 6.9.degree. C.), dimethyl sulfide (b.p. 37.3.degree. C.), methyl mercaptane (b.p. 5.8.degree. C.), formic acid (b.p. 100.75.degree. C.), formic acid methylester (b.p. 31.5.degree. C.), formaldehyde (b.p. -21.degree. C.), formaldehyde dimethyl acetale (b.p. 45.5.degree. C.) or acetic acid methylester (b.p. 56.95.degree. C.), as well as solubilities and vapor pressures are very different and since the intensity of odour of the individual compounds is also very different and in addition numerous azeotropic mixtures are formed, the object of obtaining highly pure dimethylether in high yield by a more economical process compared to the state of the art, is very difficult to achieve.
The investigations of applicant, which have been carried out during several years in numerous test sequences, in laboratory, pilot plant and technical unit, have non-obviously led to the result that highly pure dimethylether can be produced nearly quantitatively by the inventive catalysts.
The inventive catalysts are catalysts of the .gamma.-Al.sub.2 O.sub.3 -type, which contains in contrast to the catalysts of the state of the art, only very small quantities of SiO.sub.2. The inventive catalysts lead to considerably better results than the known catalysts. The catalysts contain 0.0001 to &lt;1 weight-% of SiO.sub.2. The preferred SiO.sub.2 -concentration is 0.001 to 0.5 weight-% and a particularly preferred concentration is 0.001 to 0.2 weight-% SiO.sub.2.
Additionally the inventive catalysts may contain other components in small and very small quantities, for example Na.sub.2 O or other alkali- and alkaline earth oxides, alkali, alkaline earth- or aluminum sulfate, iron oxide, cobalt oxide, nickel oxide and other compounds.
The reaction in the presence of the inventive catalysts is carried out at a temperature of 140.degree. to 500.degree. C., preferably at 150.degree. to 450.degree. C. and pressure of 1 to 50 bar, preferably of 1 to 25 bar. The reaction may be carried out in the gas or liquid phase, preferably in the gas phase. Preferably the pressure in the synthesis reactor and in the distillation columns where the crude reaction product is worked up are adjusted to each other. Operation is carried out at a liquid hourly space velocity (LHSV) of 0.2-16 1/l h, preferably of 0.5-13.5 1/l h.
The inventive process can be operated discontinuously however preferably continues.
Reactors, which can be used for the inventive process may be the known reactors of the state of the art, like fixed bed-, fluid bed- or fluidized bed-reactors, but also modified, new, and improved reactors, which are suited for catalytic reactions.
The reaction in the presence of catalysts, resp. inventive catalysts can be kinetically or thermo-dynamically controlled, dependent on the reaction parameters. As a consequence the corresponding quantities of dimethylether are obtained at the reactor exit besides the respective unconverted quantities of methanol.